Verbs are either active or passive in voice. In the active voice, the subject and verb relationship is straightforward: the subject is a do-er. In the passive voice, the subject of the sentence is not a do-er. It is shown with by + do-er or is not shown in the sentence. Passive voice is used when the action is the focus, not the subject. It is not important (or not known) who does the action. Example :
1. The window is broken. (It is not known who broke the window, or it is not important to know who broke the window.)
2. class has been canceled. (The focus is on the class being canceled. It is not important to know who canceled it.)